1. Field of the Invention
The invention relates to a method and a device for locating moving objects, having the characteristic features of the preamble of claim 1 or of claim 10. The invention further relates to applications of the method or the device.
2. Description of Related Art
For locating moving objects, arrangement of so-called transponders on the objects is known, these transponders being able to communicate with an interrogator arranged in a stationary position on a path of movement of the object. Here, transponders, chip cards and labels are grouped under the term transponder (often also called “tag”) within the scope of the present invention. The interrogator is a transceiver facility with at least one antenna, which transmits an electromagnetic signal or field within a reading range. As soon as the object with the associated transponder comes into the reading range of the interrogator and an antenna of the transponder receives the electromagnetic signal transmitted by the interrogator, a transmitter of the transponder is stimulated to emit a response signal. This in turn is detected, decoded and converted by a receiver of the interrogator. Usually the response signal from the transponder is a signal modulated with the interrogator's transmitted signal according to a mathematical relation. In applications where, in addition to simple position determination for the moving objects, an identification and subsequent sorting of the objects is also required, superposing the response signal on information personalizing the transponder is also known. In this way the interrogator receives information not only about the positions of the objects, but also about their individual order.
A problem with this method is that the transponder's response signal is dependent on an individual response sensitivity and a spatial orientation of the transponder relative to the interrogator antenna. This makes the locating of the transponder and/or the objects imprecise. This effect can lead to objects close beside one another not getting resolution, so that their order is incorrectly detected and sorting errors then follow.
A first approach to solving this problem involves evaluating the amplitude of the reverse modulated response signal from the transponder. For this, the peak value of an amplitude characteristic is usually assigned to the spatial center of the interrogator antenna's reading range. However, since depending on the transponder's orientation the amplitude peak value need not necessarily match the spatial center of the interrogator's reading range, this too can lead to imprecise position determination and sorting errors. For this reason it is not possible to distinguish transponders closely following one another.